1. Field of the Invention
The present invention relates to a technique of processing the medical images captured by various types of medical image acquisition apparatuses (modalities).
2. Description of the Related Art
Patent literature 1 (Japanese Patent Laid-Open. No. 2010-88627) discloses a PAT (Photoacoustic Tomography) apparatus. The PAT apparatus is an apparatus which excites an absorbing substance in a sample by irradiating a measurement target with optical pulses, and detects the photoacoustic signal generated by the thermoelastic expansion of the absorbing substance, thereby imaging properties associated with the light absorption of the measurement target. That is, the apparatus images an optical energy deposition amount distribution (optical energy absorption density distribution) in an object with respect to irradiation light. In addition, based on this distribution, the apparatus images the light absorption coefficient distribution of the object concerning an irradiation wavelength. In addition, it is possible to image the density distributions of substances constituting a living body based on light absorption coefficient distributions concerning a plurality of wavelengths. These images are expected to visualize information associated with a new blood vessel generated around a malignant tumor such as breast cancer. These images will be collectively referred to as photoacoustic tomographic images (PAT images) hereinafter.
PAT is designed to irradiate a human body with near-infrared pulses of low energy, and hence has difficulty in imaging a deep portion of the human body as compared with X-rays. Under the circumstance, according to patent literature 1, the PAT apparatus designed to measure breasts is used in a form such that a breast is held by two flat plates (to be referred to as holding plates hereinafter) and imaged while the thickness of the breast is reduced.
An image matching method is used as a method of aligning a PAT image with an MRI image. For example, non-patent literature 1 (Angela Lee, et al., “Breast X-ray and MR image fusion using finite element modeling”, Proc. Workshop on Breast Image Analysis in conjunction with MICCAI 2011, pp. 129-136, 2011) discloses a technique for alignment between an X-ray mammography (MMG) image obtained by imaging a breast compressed by flat plates and an MRI image of the breast as in the case of PAT. Non-patent literature 2 (Iwashita, et al., “2D-3D Registration. Using 2D Distance Maps” in Meeting on Image Recognition and Understanding 2005 (MIRU 2005)) discloses a technique for high-speed alignment between a three-dimensional shape model of an object and a silhouette image of the object. In addition, patent literature 2 (Japanese Patent Laid-Open. No. 2008-235504) discloses a technique of detecting a part assembly error by comparing two-dimensional shape data recognized from a camera image of an assembly part with two-dimensional shape data obtained by projecting three-dimensional shape data of the assembly part in the viewpoint direction of the camera. Furthermore, non-patent literature 3 (C. Tanner, et al., “Breast Shapes on Real and Simulated Mammograms”, Proc. Int. Workshop on Digital Mammography 2010 (IWDM 2010), LNCS 6136, pp. 540-547, 2010) discloses a technique of evaluating the shape of a breast after deformation which is obtained as a result of performing a physical deformation simulation by compression with flat plates with respect to an MRI image based on the two-dimensional shape of the breast which is extracted from an MMG image.
In addition, as described in patent literature 1, the PAT apparatus calculates an optical energy deposition amount distribution in an object from a detected photoacoustic signal. Since an optical energy deposition amount distribution is represented by the product of an absorption coefficient (μa) and a reaching light amount (Φ), a light absorption coefficient distribution is calculated by dividing the optical energy deposition amount distribution by a light amount distribution. Since a light amount distribution is a three-dimensional distribution of light amounts in an inner region of an object, this calculation requires the three-dimensional shape of the object which is used to specify the inner region of the object. For example, according to patent literature 1, a simple three-dimensional shape of an object is acquired based on the two-dimensional measurement range set on the plane of each holding plate which holds a breast as an object and the region defined by the distance (the thickness of the object) between the two holding plates.
Since PAT and MRI images differ in their characteristics to be imaged, all the structures depicted in the MRI image do not match those on the PAT image. It is therefore difficult to execute high-accuracy alignment.